Fill a tiki mug to the top with crushed ice
Pour 1 oz of lime juice, 1.5 oz of grapefruit juice, 1 oz of cinnamon syrup, 1.5 oz of dark rum and 1 oz of overproof rum into a shaker
Fill the shaker with ice cubes and shake
Strain into the tiki mug
Garnish with a grapefruit wedge and a mint sprig